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Exporter

Section: Perl Programmers Reference Guide (3)
Updated: perl 5.004, patch 04
Index

NAME

Exporter - Implements default import method for modules

SYNOPSIS

In module ModuleName.pm:

  package ModuleName;  require Exporter;  @ISA = qw(Exporter);

  @EXPORT = qw(...);            # symbols to export by default  @EXPORT_OK = qw(...);         # symbols to export on request  %EXPORT_TAGS = tag => [...];  # define names for sets of symbols

In other files which wish to use ModuleName:

  use ModuleName;               # import default symbols into my package

  use ModuleName qw(...);       # import listed symbols into my package

  use ModuleName ();            # do not import any symbols

DESCRIPTION

The Exporter module implements a default import method whichmany modules choose to inherit rather than implement their own.

Perl automatically calls the import method when processing ause statement for a module. Modules and use are documentedin the perlfunc manpage and the perlmod manpage. Understanding the concept ofmodules and how the use statement operates is important tounderstanding the Exporter.

Selecting What To Export

Do not export method names!

Do not export anything else by default without a good reason!

Exports pollute the namespace of the module user. If you must exporttry to use @EXPORT_OK in preference to @EXPORT and avoid short orcommon symbol names to reduce the risk of name clashes.

Generally anything not exported is still accessible from outside themodule using the ModuleName::item_name (or $blessed_ref->method)syntax. By convention you can use a leading underscore on names toinformally indicate that they are `internal' and not for public use.

(It is actually possible to get private functions by saying:

  my $subref = sub { ... };  &$subref;

But there's no way to call that directly as a method, since a methodmust have a name in the symbol table.)

As a general rule, if the module is trying to be object orientedthen export nothing. If it's just a collection of functions then@EXPORT_OK anything but use @EXPORT with caution.

Other module design guidelines can be found in the perlmod manpage.

Specialised Import Lists

If the first entry in an import list begins with !, : or / then thelist is treated as a series of specifications which either add to ordelete from the list of names to import. They are processed left toright. Specifications are in the form:

    [!]name         This name only    [!]:DEFAULT     All names in @EXPORT    [!]:tag         All names in $EXPORT_TAGS{tag} anonymous list    [!]/pattern/    All names in @EXPORT and @EXPORT_OK which match

A leading ! indicates that matching names should be deleted from thelist of names to import. If the first specification is a deletion itis treated as though preceded by :DEFAULT. If you just want to importextra names in addition to the default set you will still need toinclude :DEFAULT explicitly.

e.g., Module.pm defines:

    @EXPORT      = qw(A1 A2 A3 A4 A5);    @EXPORT_OK   = qw(B1 B2 B3 B4 B5);    %EXPORT_TAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]);

    Note that you cannot use tags in @EXPORT or @EXPORT_OK.    Names in EXPORT_TAGS must also appear in @EXPORT or @EXPORT_OK.

An application using Module can say something like:

    use Module qw(:DEFAULT :T2 !B3 A3);

Other examples include:

    use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);    use POSIX  qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);

Remember that most patterns (using //) will need to be anchoredwith a leading ^, e.g., /^EXIT/ rather than /EXIT/.

You can say BEGIN { $Exporter::Verbose=1 } to see how thespecifications are being processed and what is actually being importedinto modules.

Exporting without using Export's import method

Exporter has a special method, `export_to_level' which is used in situationswhere you can't directly call Export's import method. The export_to_levelmethod looks like:

MyPackage->export_to_level($where_to_export, @what_to_export);

where $where_to_export is an integer telling how far up the calling stackto export your symbols, and @what_to_export is an array telling whatsymbols *to* export (usually this is @_).

For example, suppose that you have a module, A, which already has animport function:

package A;

@ISA = qw(Exporter);@EXPORT_OK = qw ($b);

sub import{
$A::b = 1; # not a very useful import method}

and you want to Export symbol $A::b back to the module that called package A. Since Exporter relies on the import method to work, via inheritance, as it stands Exporter::import() will never get called. Instead, say the following:

package A;@ISA = qw(Exporter);@EXPORT_OK = qw ($b);

sub import{
$A::b = 1;
A->export_to_level(1, @_);}

This will export the symbols one level `above' the current package - ie: to the program or module that used package A.

Note: Be careful not to modify `@_' at all before you call export_to_level- or people using your package will get very unexplained results!

Module Version Checking

The Exporter module will convert an attempt to import a number from amodule into a call to $module_name->require_version($value). This canbe used to validate that the version of the module being used isgreater than or equal to the required version.

The Exporter module supplies a default require_version method whichchecks the value of $VERSION in the exporting module.

Since the default require_version method treats the $VERSION number asa simple numeric value it will regard version 1.10 as lower than1.9. For this reason it is strongly recommended that you use numberswith at least two decimal places, e.g., 1.09.

Managing Unknown Symbols

In some situations you may want to prevent certain symbols from beingexported. Typically this applies to extensions which have functionsor constants that may not exist on some systems.

The names of any symbols that cannot be exported should be listedin the @EXPORT_FAIL array.

If a module attempts to import any of these symbols the Exporterwill give the module an opportunity to handle the situation beforegenerating an error. The Exporter will call an export_fail methodwith a list of the failed symbols:

  @failed_symbols = $module_name->export_fail(@failed_symbols);

If the export_fail method returns an empty list then no error isrecorded and all the requested symbols are exported. If the returnedlist is not empty then an error is generated for each symbol and theexport fails. The Exporter provides a default export_fail method whichsimply returns the list unchanged.

Uses for the export_fail method include giving better error messagesfor some symbols and performing lazy architectural checks (put moresymbols into @EXPORT_FAIL by default and then take them out if someoneactually tries to use them and an expensive check shows that they areusable on that platform).

Tag Handling Utility Functions

Since the symbols listed within %EXPORT_TAGS must also appear in either@EXPORT or @EXPORT_OK, two utility functions are provided which allowyou to easily add tagged sets of symbols to @EXPORT or @EXPORT_OK:

  %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);

  Exporter::export_tags('foo');     # add aa, bb and cc to @EXPORT  Exporter::export_ok_tags('bar');  # add aa, cc and dd to @EXPORT_OK

Any names which are not tags are added to @EXPORT or @EXPORT_OKunchanged but will trigger a warning (with -w) to avoid misspelt tagsnames being silently added to @EXPORT or @EXPORT_OK. Future versionsmay make this a fatal error.

Index

NAME
SYNOPSIS
DESCRIPTION
Selecting What To Export
Specialised Import Lists
Exporting without using Export's import method
Module Version Checking
Managing Unknown Symbols
Tag Handling Utility Functions

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